The objectives are, first, to discover when the vascular pressure in a segment wedged by a balloon catheter is influenced by the effects of the alveolar or pulmonary venous pressure. This is to be studied in the open and closed chested dog with pulmonary edemas of the hemodynamic and "capillary leak" types. The effect of this edema on the compliance and resistance of the arterial, microvascular and venous segments will be studied using very thin retrograde catheters which measure side pressures of the small vessels within the lung. The second objective is to study the relative significance of factors such as the fall in venous return and the increased vascular resistance of the lung in causing the fall in cardiac output which occurs with positive alveolar pressures. The lung volume will be kept constant by bilateral pneumothoraces pressurized by the same amount as the increase in alveolar pressure. These cardiac output changes caused by intrathoracic pressures will be contrasted with those associated with volume alterations at the same intrathoracic pressure. If the factors affecting wedge pressures during mechanical ventilation are understood, it may be possible to check the veracity of such measurements when used in the management of patients with ARDS. It is also important to know the effect on the cardiac output of such patients of volume changes, which depend on the compliance and recoil, versus alveolar pressure changes which only depend on the ventilator pressures.